I've been bitten by poor key distribution in tests in the past, so this is spot on: 'I'd run it with Zipfian, Pareto, and Dirac delta distributions, and I'd choose read-modify-write transactions.'

And of course, a dataset bigger than all combined RAM.

Also: http://smalldatum.blogspot.ie/2014/04/biebermarks.html -- the "Biebermark", where just a single row out of the entire db is contended on in a read/modify/write transaction: "the inspiration for this is maintaining counts for [highly contended] popular entities like Justin Bieber and One Direction."